Such optical couplers are disclosed in the article of C. P. Hussel et al entitled "Adiabatic invariance in GRIN channel waveguides and its use in 3-dB cross couplers" published in Applied Optics, Volume 29, Number 28, (1990), pages 4105 to 4110 (FIG. 5). The function of the input and output waveguides can be transposed as described in U.S. Pat. No. 4,850,666. This patent discloses that such a branching device can be driven in both directions. The propagation constant is in each case determined by the width of the waveguides. All waveguides are joined together in a crossover region. The redirection of the waveguides to a parallel path is shown only schematically. Measures for the transition to unitary waveguide dimensions at the connections of the integrated-optical component are not described.
Tervonen et al discloses a Mach-Zehnder interferometer in the article entitled "Channel waveguide Mach-Zehnder interferometer wavelength splitting and combining" published in the Proceedings SPIE 1513, (1991), pages 71 to 75 (FIG. 1). For an arrangement corresponding to the above-mentioned U.S. Pat. No. 4,850,666, Tervonen et al disclose in this article that the two waveguides having the same widths are guided in S-shape arcuate segments to the coupling location.
The typical crossover angles are very acute and are typically less than 0.2.degree.. The tips of the material surrounding the waveguides and which occur in the crossover region cannot be ideally produced. A blunt end portion of the tip having a width of at least 0.5 .mu.m cannot be avoided in waveguides buried in glass and produced utilizing photolithography and ion exchange. Relatively high losses are produced in this way.
In the arrangement disclosed in U.S. Pat. No. 4,961,619, a blunt tip is, however, expressly introduced for a symmetrical 2.times.2 optical coupler having a relatively large crossover angle of from 5.degree. to 10.degree.. Because of this blunt tip and in combination therewith, all waveguides having the same width taper in the crossover region whereby overall losses are reduced.
An asymmetrical 2.times.2 coupler without crossover and without contact is disclosed in the article of Takagi et al entitled "Wavelength Characteristics of (2.times.2) Optical Channel-Type Directional Couplers with Symmetric or Nonsymmetric Coupling Structures" published in the Journal of Lightwave Technology, Volume 10, Number 6, (June 1992), pages 735 to 746 (FIG. 7).
In this publication, an arrangement is disclosed wherein waveguides having the same width are brought together from circular arcuate segments via S-curved segments into an interaction region where the waveguides are almost parallel. One waveguide tapers here and is, after an S-curve segment, brought with a taper again to the normal width. The curve in the narrow waveguide leads to increased attenuation and reduces the spectral bandwidth.
A variation described as uniformly asymmetric by Takagi et al is also disclosed in the article of Chen entitled "Wavelength-Selective Asymmetric Integrated Optical Couplers with Variable Spacing between the Waveguides" published in "Frequenz", Volume 45, (1991), pages 225 to 232. Two waveguides are brought into S-curve segments at a minimal mutual spacing and, after a short parallel segment, are again separated by S-curve segments. Chen discloses kinks in lieu of curves. The two waveguides each have the same width over its entire length but these widths are different with respect to each other. The two waveguides can also have refractive indices which are the same along the entire length thereof but different with respect to each other.